The present disclosure relates generally to photolithography and, more particularly, to an exposure apparatus and method used in fabrication of a semiconductor device.
A scanner, also known as a step-and-scan system, is a type of exposure tool used in modern-day photolithography processes to fabricate semiconductor devices. A scanner provides for moving a substrate (e.g., a wafer) and reticle (commonly referred to as a “mask”) with respect to one another while exposing photosensitive material present on the substrate. Conventional scanners, as well as other conventional exposure tools, are limited in that the exposure process is executed at a fixed focus length for a given exposure field. An exposure field includes an area of a substrate covered (e.g., exposed) by a single exposure or “shot.” In contrast to this, today's semiconductor devices often include dramatic pattern density differences providing for great variations in feature height above the plane of the substrate. For example, a dual damascene process, a typical method of forming interconnects in a semiconductor device, provides a large step height difference that can result in a large intra-exposure field focus range. This intra-field focus range negatively impacts the depth-of-focus (DOF). A poor DOF provides for decreased resolution in the photolithography process.
As such, an improved exposure apparatus and method are desired.